Nail biting can be a result of boredom or stress. It affects about 20-30% of the population but can be unhealthy and harmful. When you chew on your nails you increase the risk of cracking, chipping or wearing the incisal edges of your teeth. When you position your lower jaw forward for extended periods of time, you can impact your jaw and increase your risk for TMJ dysfunction. You are also more prone to illness and possible infections of your fingers.